The invention relates to an automated system for navigation control and more particularly to the use of non-linear prediction mechanisms used in conjunction with a predictive controller.
Automatic systems for ship control have been in existence for several years. These systems are being utilized for forward speed control, course-keeping, rudder roll stabilization, and dynamic ship positioning. Existing autopilot systems are mainly used to maintain a heading (longitudinal axis) of a ship in a predefined direction. Environmental disturbances, such as wind and water currents, may cause the ship to move in a direction that is several degrees off its intended heading. As a result, these autopilots are not capable of accurately keeping the ship on a predefined track, which is critical for navigation in restricted waterways. The impetus behind the development of a track-keeping autopilot stems from confined waterway navigation in which there is little margin for navigational errors. The development of a track-keeping autopilot became feasible with the advent of the Differential Global Positioning System (DGPS).
Predictive controllers are a well known form of controller and their use in the process control industry is well established. However, many predictive control methods require an analytical, closed form linear prediction model to formulate the control law. Such models can be somewhat restrictive in the ability to accurately represent a process and thus, the ensuing control law may not perform as required.
It has now been found that a modified generalized predictive controller using an external correlation processor for performing prediction can be implemented resulting in excellent control results even in substantially non-linear environments such as ship navigation and control.
The ASAS is an advanced track-keeping autopilot. It combines the predictive capabilities of a correlation based Ship Predictor System (SPS) with advanced predictive Control technology to render a versatile and accurate track-keeping autopilot. ASAS allows, for the first time, ship autopiloting in confined waterways since it is capable of maintaing a ship within a few meters from a pre-set track.
According to the invention there is provided a method of navigation control for a vessel comprising the steps of:
(a) providing a correlation processor for determining according to a non-linear correlation a set of predictions of vessel motion based on a set of sensory input values;
(b) determining from the predictions and from actual vessel motion a control law of ship motion;
(c) using the control law, forming a predictive controller for providing a control signal indicative of navigation control; and,
(d) at intervals updating the predictive controller based on another control law formed according to step (b).